Piston Balancing

[andyelan]

Hi Everyone

Could anyone out there suggest where is the best place to remove metal when balancing a set of Twin Cam pistons.

Regards
Andy

[ceejay]

on the inside of the lower skirt by machining in a lathe, (Less is best) or small indent drilling's under neath the gudgeon pin boss. But you should not need to do this as most performance pistons are already balanced for road use.. race items will require finer balance weight of all pistons though.
CC

[rgh0]

If taking material off an alternative is to gind a little from the inner ends of the gudgeon pins rather than the piston itself. If removing material from the piston gudgeon pin bosses do it at the inside edge by drill small indents as suggested by Colin or beveling the inside edge. Do not remove materal near where the boss joins the piston skirt.

cheers
Rohan

[ceejay]

Yes that's right, thanks Rohan for adding that. Balancing is a slow and very time consuming job, use a very accurate digital scale, and use the lightest piston as the weight bench mark to balance the others to... good luck and enjoy.
Col.

[rcraven]

What do you expect to achieve by going to great lengths to get every piston weighing the same? Even if each one is exactly the same mass, the forces on each one will be considerably different at any one time and constantly varying, depending on what part of the 4 stroke cycle it is going through and how far it is from the clutch end of the crank, and of course the crank itself will be twisting.

[msd1107]

Robert,

Balancing the pistons to close tolerances versus the standard tolerances results in a much smoother engine. It needs to be experienced to believe it.

When you consider the piston is subjected to peak accelerations of several thousand Gs, even a fraction of a gram imbalance results in substantially different forces being reacted into the engine structure from the 1-4 or 2-3 pistons, or the linear part of the 1-2-3-4 pistons.

You should also do the same thing to the connecting rods, both overall weight and small end and big end.

David
1968 36/7988

[rcraven]

I can see why very small differences in weight make a difference, but why do think that what you should aim for is having them all exactly the same when at any given time the forces on each piston will differ so much? Having exactly equal weight pistons won't mean that the opposing vibrations or forces are balanced out, especially if you concentrate on reducing them all to the weight of the lightest piston you happen to have without regard to what the best weight might be for all the characteristics of your engine including your particular crankshaft. (And then we get onto to the question whether it is possible or desirable to have a fully balanced crankshaft.)

[andyelan]

Hi Everyone

Just to clarify things. The pistons I have are new, standard size OE, but they're all individual ones not a set and as such their weights are outside the book limits. As I am simply building the engine up for road use, not racing, and as I've no intention of touching the rods or crank, all I intend to do is bring them within the 4 gramms specified in the workshop manual (mybe I'll aim for 3 gramms).

On the general subject of balancing though, my Europa has a fully balanced but otherwise standard Big Valve and the improvement over the standard engine is amazing. When time comes for a full rebuild of the Plus 2 engine, whatever else I do, it will get fully balanced

Andy

[bengalcharlie]

Absolutely, engine balancing is the most important step you can take in order to build a good engine. It makes such a difference and is far more important than spending thousands on hot cams, race pistons....
You can put all these goodies on an unbalanced engine and you will end up with a pig!
Robert, Just accept this as a fact, engine rebuilders and racers all over the world agree on this so not point trying to outsmart the specialists!

Cheers
Robin

[gerrym]

Random but true:- Due to coupling bolts being dropped over the side, had to balance up a mis-matched set. Achieved +/- 0.5 grams per bolt/nut. Mind you this was using the lab Metler, 7MW drive, 9500RPM. Yes balance does make a difference.

Gerry

[rgh0]

The principles of balancing a flat plane crank 4 cylinder are relatively simple.

1. Balance the crank. flywheel, and clutch cover individually and as an assembly for no out of balance rotating mass. - i.e the counterweights match the journals weights and any unbalance in the flywheel and clutch cover is removed and when assembled any unbalance due to any slight non concentric assembly is corrected for. This balancing especially needed if you have machined the journals during a rebuild even in a road engine. Cost of getting this done at a machine shop with spin balancing equipment for the 3 components and the assembly is around A$150 to A$200.

2. Match balance the masses of the pistons and the end weights of the con rods. The 2 piston rod assemblies going up and 2 going down balance each other if weights are matched. (Well at least they balance for all but the secondary out of balance couple). I match rod ends and pistons to be all within 1 gram - if your bothering doing it, its not much harder to get within 1 gram than 3 to 4.

Larger capacity 4's ( eg the Porsche 944 and older 4 cylinder SAABs, the larger Mitsubishi 4's ) added balance shafts that spin at twice engine speed that are setup to balance out the secondary couple unbalance. It is this inherent unbalance in a 4 that gives the twink its high levels of vibration in the nose of the egnine at high revs. Lighter rods and pistons minimise this unbalance. These days most makers of large 4's work on stiff blocks and light pistons and rods rather than balance shafts.

cheers
Rohan

[bill308]

Thanks Rohan. I was not quite clear on what the requirements were for crank balancing.

If I understand correctly, there is an optimum sequence to be followed here.

1. Balance the rods by weighing both ends with nuts/bolts but no mechanical locking device.
2. With the big end weights known, the equivalent (bobb?) weights can be added to the crank journels and the crank can be dynamically balanced along with any locating pins.
3. Attach the flywheel to the crank and dynamically balance the flywheel.
4. Attach the clutch cover (without driven plate) to the flywheel and dynamically balance it.
5. Piston assemblies are balanced separately. Are rings included, perhaps after end gapping?

Is this correct?

In the past, I've asked the balancer to try for 0.1 gm.

Again, in the past, I torque up the rod bolts with a drop of Loctite in lieu of any mechanical locking device.

Bill

[bill308]

One other thing I forgot to add is that whitness marks should also be added. This insures that the assembly can be assembled with the correct relationship to each other. A simple prick punch at the periphery should suffice.

Bill

[denicholls2]

2. Match balance the masses of the pistons and the end weights of the con rods. The 2 piston rod assemblies going up and 2 going down balance each other if weights are matched. (Well at least they balance for all but the secondary out of balance couple). I match rod ends and pistons to be all within 1 gram - if your bothering doing it, its not much harder to get within 1 gram than 3 to 4.

Yes, when I read 4 grams, I immediately thought "well, that's what the shop manual says because you surely don't want them any worse." But it's easy to do a lot better, and count me in the camp that says better will be noticed.

As to the ideal weight for pistons and rods, because they have to completely reverse direction at up to 15,000 (steel crank maybe more) times a minute, it would be as close to zero as practical while retaining the other characteristics necessary to be a piston and connect same to a rapidly spinning crankshaft. So no, you will never lose setting the piston weight to the lowest common denominator unless you do your shaving badly.

[andyelan]

Hi There

Thanks everyone for the info

Regards
Andy